A major factor in the usefulness of a CT scanner is the speed and rapidity with which it performs its scanning function. Although it is now commonplace to perform a scan of a single transaxial cross-section of a patient's internal organs in two seconds or less, a complete study of a volume of interest that includes on the order of 20 high energy scans typically consumes 30 minutes or more. The vast portion of this is idle time to permit the x-ray tube to cool down between scans to avoid damaging the tube. Even with the usual precautions, however, x-ray tubes fail frequently in heavy use, resulting in temporary shut-down of the scanner.
As is well known, x-rays may be generated in a vacuum tube that comprises an anode and a cathode generally referred to as an electron gun which in turn includes a heatable tungsten filament connected to a high voltage source adapted for emitting a high energy beam of accelerated electrons. The anode is in the form of a metal target displaced a short distance from the cathode to stop the accelerated electron beam. The impact, through a relatively inefficient process, generates x-rays. The X-rays, also known as Bremsstrahlung or braking radiation, are produced by the deceleration of the electrons as they pass near a tungsten nucleus. Since typically less than one percent of the total energy of the accelerated electrons is converted to electromagnetic radiation, the bulk of the energy created by the high voltage source on the cathode is converted to thermal energy at the target area.
To minimize the debilitating effects of this resultant heat effect in conventional, fixed anode x-ray tubes, the anode is generally provided with a through flow of cooling fluid to help dissipate the heat. Nonetheless, the generation of considerable heat at a fixed focal spot creates gross limitations on the energy output capacity of the tube as well as on its limits of continuous operability.
A significant improvement was achieved by the rotating anode x-ray tube which expanded the focal spot on the target from a point to a circle. At first, such rotating anode tubes relied on radiation for heat dissipation; however, this too, quickly proved to be limiting. Although efforts for providing through flow cooling were suggested, such as for example, by Fetter in U.S. Pat. No. 4,309,637, rotating type tubes created a new set of problems. As described in the Fetter patent, the evacuated region of the tube must be sealed to maintain the necessary vacuum. Since the shaft of the anode must be provided with mechanical means for rotation, bearings must be provided within the sealed region necessitating the need to use relatively small bearings devoid of normal lubrication. This has resulted in a new failure mode for such tubes.
These problems are particularly exacerbated when the tube is intended as a mobile x-ray source such as in a rotational type CT scanner where it is impractical to utilize a mechanical pump for continuous maintenance of a high vacuum region while the invention will be described particularly in connection with rotational CT scanner application, it will be appreciated that the X-ray tube is useful in a variety of X-ray settings, such as, for example, X-ray diffraction applications and digital X-ray imaging.